Convergence of (n!)^2 / (kn)! for Positive Integers k: Ratio Test Explanation

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In summary, the conversation discusses the use of the ratio test to determine the convergence of a series involving factorials. The question asks for which positive integers k the series is convergent, and the participants discuss the assumptions and limitations of using the ratio test. One participant points out that the series is not alternating, and another mentions that the ratio test is inconclusive only when the limit is 1 or doesn't exist.
  • #1
remaan
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The hardest ever,,,

Homework Statement



40) for which positive intigers k is the following sries convergent:
(n!) ^2 / (kn)!

Homework Equations



using the ratio test the limit should be less than 1

The Attempt at a Solution



I tried the ratio test, but I got inifinty?
 
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  • #2
Hi remaan! :smile:
remaan said:
for which positive intigers k is the following sries convergent:
(n!) ^2 / (kn)!

using the ratio test the limit should be less than 1

I tried the ratio test, but I got inifinty?

Hint: which is larger, (5!)2 or 10! ? :wink:
 
  • #3


remaan said:
using the ratio test the limit should be less than 1
No -- it could be less than 1, but it doesn't have to. You're making some assumptions when you said that; what are they? (Or, I suppose you could have just made a mistake)

I tried the ratio test, but I got inifinty?
What does that tell you?
 
  • #4


tiny-tim said:
Hi remaan! :smile:


Hint: which is larger, (5!)2 or 10! ? :wink:

Hi,

mm, 1o! is greater, but how can I benifit from this in finding K ??
 
  • #5


Hurkyl said:
No -- it could be less than 1, but it doesn't have to. You're making some assumptions when you said that; what are they? (Or, I suppose you could have just made a mistake)

I mention that because the question says "positive intergers,"


What does that tell you?

It tells that the series is not alternating and I can't ( ratio test ) to find the k .
 
  • #6


remaan said:
I can't ( ratio test ) to find the k .
Why not? The only time the ratio test is inconclusive is when the limit is 1, or doesn't exist, and for all possible values of k, you're not in either of those cases
 

1. What is the ratio test and how does it apply to the convergence of (n!)^2 / (kn)! for positive integers k?

The ratio test is a method used to determine the convergence or divergence of a series. In the case of (n!)^2 / (kn)!, we can use the ratio test by taking the limit as n approaches infinity of the absolute value of the ratio of the (n+1)th term to the nth term. If the limit is less than 1, the series converges. If the limit is greater than 1, the series diverges. If the limit is equal to 1, the test is inconclusive and another method must be used.

2. How does the factorial in the numerator affect the convergence of the series?

The factorial in the numerator, (n!)^2, grows much faster than the factorial in the denominator, (kn)!, as n increases. This means that the terms in the series decrease rapidly, leading to the series converging.

3. Can the ratio test be used for other types of series?

Yes, the ratio test can be used for other types of series, as long as the terms in the series are positive and approach 0 as n approaches infinity. However, it may not always be the most efficient method for determining convergence.

4. Does the value of k affect the convergence of the series?

Yes, the value of k does affect the convergence of the series. As k increases, the terms in the series become larger, making it more difficult for the series to converge. However, as long as k is a positive integer, the series will still converge due to the factorial in the numerator.

5. How can we prove the convergence of (n!)^2 / (kn)! for positive integers k?

To prove the convergence of (n!)^2 / (kn)! for positive integers k, we can use the ratio test as described in question 1. We can also use other methods such as the comparison test, where we compare the series to a known convergent or divergent series. Additionally, we can use mathematical induction to show that the series converges for all values of n.

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